Flat display panels having excellent features such as radiation free and high definition have dominated the market. Common flat displays include liquid crystal displays, plasma displays, organic electroluminescent displays, and the like. By using the most common liquid crystal displays as an example, the liquid crystal display is mainly formed by a pixel array substrate, a color filter substrate, and a liquid crystal layer sandwiched between the two. On the conventional pixel array substrate, thin film transistors are mostly used as switching elements of pixel structures, and therefore, the performance of the liquid crystal display depends on the quality of the thin film transistors. In other words, the conductivity of a channel layer in the thin film transistor greatly affects the quality of the thin film transistor and the performance of the liquid crystal display.
On the other hand, in the process of manufacturing the thin film transistor, four masks are needed to respectively pattern a gate electrode, an oxide semiconductor, an etching stop layer and a source electrode/drain electrode. However, the masks are expensive, and using a large number of masks to manufacture the thin film transistors will increase the overall manufacturing cost. Therefore, how to reduce the number of the masks as well as improve the quality of the thin film transistor becomes a task for the liquid crystal display industry that is in urgent need to be solved.